1. Technical Field
This invention relates generally to electronic copiers, and more particularly to such copiers capable of image processing such as for example by object isolation by referencing the background of an image being copied.
2. Background Art
Object isolation of images being electronically copied often employs image segmentation by a thresholding technique wherein the image is partitioned into a finite number of disjoint (non-overlapping) picture elements, called pixels, each of which is represented as a pure binary quantity, i.e., object or background. Thresholding is particularly useful for images containing solid area objects resting upon a contrasting background. When using a threshold rule for image segmentation of positive images, one assigns all pixels at or above the threshold reflectance to the background. All pixels with reflectance below threshold fall inside the object.
It is desirable that the threshold reflectance be determined by an examination of the background level of the entire original document. So called "background referencing" techniques, wherein estimates of the background level are periodically updated so that the threshold reflectance is constantly being updated, are generally not suitable for document copiers because of the high incidents of artifacts produced.
Accordingly, it is generally desirable to select one threshold reflectance for the entire, or a substantial portion of the entire, original document. This of course requires that the reflectance levels over the whole document be considered before the image signal can be processed.
In the prior art, this is accomplished by pre-scanning the document to gather sufficient data regarding the various reflectances across the image so that a threshold reflectance can be determined. During a subsequent scan, the image signal is processed according to the threshold reflectance determined using the data collected during the first scan.
A copier's life is in part limited by the physical number of times the image is scanned, and the inconvenience of the operators is measured in part by the time duration they must wait to remove the original after pressing the start button. A copier which requires only a single scan to determine the threshold reflectance and to effect image processing according to the threshold reflectance would be very preferable to a copier which requires two or more scans for the same functions.
Another drawback to copiers which require a pre-scan to determine the threshold reflectance and a second scan to process the image as the image signal comes from the scanner is that there is no provision for producing a test print of the image as processed, and for then changing the threshold reflectance before a final print without rescanning the original document.